A blowing agent is a composition, usually liquid at room temperature, which enters into a chemical reaction, here the reaction of a polyisocyanate and an isocyanate-reactive material, to create a rigid foam. The enclosed bubbles that comprise the foam contain carbon dioxide and constituents of the blowing agent in varying amounts. Yet the blowing agent is unchanged in its properties as a result of the chemical reaction; the blowing agent goes into solution as a liquid and comes out of solution as a gas during the exothermic reaction. In order to constitute a blowing agent, the material must be soluble in the constituents of the foam, but not so soluble that it destroys the polymeric linkages. It must also have a high boiling point for effective bubble formation.
For many years the blowing agents that became standard as most desirable in the preparation of isocyanate-based foams, such as rigid and flexible urethane and isocyanurate foams, were what became part of the vernacular as CFC's. Most typically, a composition identified as CFC-11 (CCl.sub.3 F) became a standard blowing agent. However, the use of this material is now banned by international treaty on the grounds that its release into the atmosphere damages the ozone layer in the stratosphere. As a consequence, this most standard blowing agent for forming isocyanate foams is now no longer available for use.
The result of the ban on CNC's led those of skill in the art to the utilization of related compositions. A solution was sought in the area of HCFC's, which are hydrogen-containing chlorofluoroalkanes. These HCFC's, and most particularly HCFC-123 (CHCl.sub.2 CF.sub.3) and HCFC-141b (CH.sub.2 ClCHClF) have relatively short lifetimes in the atmosphere and tend to decompose before they reach the earth's stratosphere which contains the ozone layer. However, as set forth, for example, in U.S. Pat. No. 5,286,759, the art has considered HCFC's to have disadvantages when compared with CFC's, and those alleged disadvantages have led persons of skill in the art to seek other CFC substitutes. In U.S. Pat. No. 5,286,759, a blowing agent was disclosed comprising a mixture of a hydrocarbon with 4 or more carbon atoms and a polyfluorocarbon component with a specific boiling point and molecular weight ratio. Exemplary of such components are polyfluorocarbon compounds, for example, perfluorocyclohexane, polyfluoroalkanes and polyfluoroethers. Finally, this patent proposes a third component, which is stated to be halocarbons, ethers, ketones, etc.
While the art thus searches for more esoteric combinations of ingredients in order to meet the need created by the forced nonuse of CFC's, it is the purpose of the present invention to provide such a CFC substitute that is composed of a specific combination of HCFC's and a hydrocarbon such as hexane or octane. The combination of HCFC's that are included in the present invention are readily available and environmentally sound. When used in the proposed combinations, they have been found to constitute satisfactory, indeed more than satisfactory, blowing agents for isocyanate resins and create a good foam resistant to decomposition and flammability.